# Spectral analysis of the spin-boson Hamiltonian with two bosons for arbitrary coupling and bounded dispersion relation

## TL;DR

This paper provides a detailed spectral analysis of the spin-boson Hamiltonian with two bosons, describing the essential spectrum structure, critical coupling values, and eigenvalue behavior under various conditions.

## Contribution

It offers an explicit description of the essential spectrum, identifies critical coupling constants, and analyzes eigenvalue accumulation for the spin-boson model with bounded dispersion relation.

## Key findings

- Essential spectrum consists of two and three-particle branches with possible gaps.
- Critical coupling constants are explicitly determined.
- Discrete eigenvalues do not accumulate at the edges of the essential spectrum.

## Abstract

We study the spectrum of the spin-boson Hamiltonian with two bosons for arbitrary coupling $\alpha>0$ in the case when the dispersion relation of the free field is a bounded function. We derive an explicit description of the essential spectrum which consists of the so-called two and three-particle branches that can be separated by a gap if the coupling is sufficiently large. It turns out, that depending on the location of the coupling constant and the energy level of the atom (with respect to certain constants depending on the maximal and the minimal values of the boson energy) as well as the validity or the violation of the infrared regularity type conditions, the essential spectrum is either a single finite interval or a disjoint union of at most six finite intervals. The corresponding critical values of the coupling constant are determined explicitly and the asymptotic lengths of the possible gaps are given when $\alpha$ approaches to the respective critical value. Under minimal smoothness and regularity conditions on the boson dispersion relation and the coupling function, we show that discrete eigenvalues can never accumulate at the edges of the two-particle branch. Moreover, we show the absence of the discrete eigenvalue accumulation at the edges of the three-particle branch in the infrared regular case.